Many rotational power sources such as motors, engines, turbines, etc. have a relationship between torque and angular speed. Accordingly, drive trains which connect such power sources to a load, such as to power a machine, drive an implement or conduct other work, are provided with different usable gear ratios. The drive train relies on the relationship between output torque and angular speed (in revolutions per minute “RPM”) to operate at a given gear ratio. Typically, the speed of the power source and/or the load determines the appropriate gear ratio for driving the load. In many systems, a drive train or transmission controller executes this selection with or without input from the operator of the machine.
The selection of gears ratios can lead to gear train oscillations, also known as shift hunting, when operating in a band around a shift point of the transmission. For example, such oscillations can occur when conditions immediately after a shift indicate that the immediately prior gear ratio should be selected instead. For example, consider a system wherein engine speed is used to determine an appropriate shift point between two gear ratios. When the shift point is reached during acceleration, i.e., increasing engine speed, the shift to a higher gear ratio will cause the engine speed to decrease to a speed lower than the shift point. This in turn, will cause the controller to execute a shift back to the lower gear. However, now the engine will unload and the engine speed will increase past the shift point, causing the controller to execute a shift back to the higher gear. Such oscillations can be inefficient and annoying, and may decrease the useful life of the drive train and the engine.
In parallel path variable transmissions, a variable direction hydrostatic element drives the gear train such as in certain split torque transmissions. One example of a hydrostatic transmission consists of a variable speed hydraulic pump and a hydraulic motor. In such a transmission, shift points result in a reversal in the direction of the variator acceleration. Thus, the shift points are necessarily defined such that the down-shift and up-shift points between any two ratios are singular, i.e. they both lie at essentially the same RPM (or transmission ratio).
One example of a system for preventing shift hunting in a parallel path variable transmission is disclosed in U.S. Pat. No. 7,660,657. The disclosed system utilizes a lock-out timer that actuates after the transmission executes a shift in order to prevent a subsequent shift until the timer has expired. However, this is a reactive approach that locks the transmission into certain ranges and can lead to rough transitions when the transmission actually shifts between ranges.